Image sensors can be designed with an array of active pixels using a semiconductor process such as CMOS. A known type of active pixel is a four transistor (4T) pixel. This type of pixel is shown in FIG. 1, and comprises a photo-sensitive element such as a photodiode PD, a buffer amplifier (source follower) sf, a reset switch res with a control line RES and a select switch sel with a control line SEL. The pixel comprises a charge conversion element, also called a floating diffusion fd. A transfer gate tr connects between the photo-sensitive element PD and the charge conversion element fd. Exposure time in a four transistor (4T) global shutter pixel is controlled by the transfer line. Before the start of the exposure period, the transfer line is pulsed when the floating diffusion is kept in its reset state. Typically, the transfer line is a horizontal line of the pixel array connected to a row of pixels in the array. This means that the exposure time will start at the same moment for all pixels connected to that same transfer line.
It is desirable, in some applications, to have a different exposure time for different regions of the image. An example application in the automotive field is described in US 2009/0225189A1 and an example of a star tracker used for navigation of spacecrafts is described in O. Yadid-Pecht, et al, “CMOS Active Pixel Sensor Star Tracker with Regional Electronic Shutter”, IEEE Journal of Solid State Circuits, vol. 32, No. 2, February 1997, pp. 285-288.
The O. Yadid-Pecht reference describes a technique to control the exposure time individually per pixel. This is applied to classical three-transistor (3T) active pixels. A series switch is connected between the reset control line and the gate of the reset transistor.